The invention will be described in the environment of an advanced definition television receiver (ADTV) of the type for processing HDTV signals proposed by the Advanced Television Research Consortium (ATRC), however it is to be understood that practice of the invention is not limited to such systems, but is applicable to other systems having harmonically related amplitude modulated carriers.
FIG. 1 illustrates a television signal in the ADTV system format. This signal is constrained to have a 6 MHz bandwidth in conformance with NTSC standards. Unlike NTSC television signal however, the ADTV signal consists of two quadrature amplitude modulated carriers, one of which is located in the lower one quarter of the 6 MHz channel space and the other which is located in the upper three quarters of the 6 MHz channel space. The upper carrier has a bandwidth which is four times as wide as the bandwidth of the lower carrier. The upper carrier frequency is exactly four times the lower carrier frequency (related to a predetermined reference). In the FIG. 1 example both carriers are modulated 16 QAM.
FIG. 2 illustrates a portion of a typical ADTV receiver apparatus including the tuner IF and QAM demodulation circuitry. A detailed description of this apparatus will not be herein provided but may be found in U.S. patent application Ser. No. 650,329 filed Feb. 4, 1991. What is to be noted however, is the parallel processing circuitry (elements 118, 120, 122, 124, 126, 128, and elements 119, 121, 123, 125, 127, and 129) for processing the two QAM signals respectively. Each of these parallel processing paths consist of relative large and complex, and therefore expensive hardware. The present invention is directed toward reducing such parallel hardware, in order to make such a system affordable to the typical consumer. More particularly the invention is directed toward utilizing processing apparatus in time division multiplexed fashion to eliminate at least a portion of the parallel processing circuitry utilized to process frequency division multiplexed signals such as the two QAM signals illustrated in FIG. 1.